IR, UV, X-Rays

Besides visible light and radio waves, light of other wavelengths is difficult or impossible to observe from Earth, because the atmosphere (thankfully, actually) adsorbs the most part of it.

Atmospheric electromagnetic opacity. Source: wikipedia

Atmospheric electromagnetic opacity. Source: wikipedia

IR: Water vapor adsorbs IR waves. Observatories must be at high altitudes to observe something in the IR range.


  • Mauna Kea observatory at more than 4,000 m over the sea level. Still, blackbodies at 300 K can radiate waves in the IR range, so the atmosphere and even the detector itself can interfere with the desired signal!
  • IRAS (Infrared Astronomy Satellite): in a 560 mile orbit, was cooled with liquid helium. Operated 10 months from 1983.
  • Spitzer Space Telescope (SST): launched in 2003, in heliocentric orbit (!). It was cooled by liquid helium which has been already exhausted, but it can still operate at some wavelengths. Check the podcast if you want.

Long wavelengths in microwave regime: The COBE (Cosmic Background Explorer), operating from 1989 to 1993, surveyed 2.7 K blackbody radiation thought to be the remnant fireball of the Big Bang.

UV: What is observable in this range? Mass loss from hot stars, and cataclysmic variable stars, and white dwarfs and pulsars.

Problems in the UV range:

  • Due to the short λ, very precise mirrors are required.
  • Glass is opaque to UV, so crystal lenses are needed.

X-Rays and Gamma Rays: Correspond to very energetic events (e.g. black holes, nuclear reactions). How are these wavelengths observed?

  • Grazing-incidence reflections (incident angle close to 90º).
  • Bragg scattering: interference produced by photon reflections from atoms in a regular crystal lattice.

Very different phenomena occur at different wavelengths, so the picture that we can see from the sky may be completely different in different wavelength ranges. Things that in the visible range are invisible, may be all shining in the UV or radio spectra. This is a good example of our Milky Way seen at different wavelengths. And THIS is a similar thing, but simpler to visualize.

Telescopes that work/worked in the X-Ray Gamma ray range:

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